Living with Liquefaction

During intense earthquake shaking, water-saturated sediments can turn into a slurry (liquefaction), causing buildings, cars and other heavy objects lying on top of what were formerly stiff, dry sediments or sand, to sink or tilt. This is something I experienced firsthand while pursing my M.Sc. degree in Geology at the University of Canterbury in 2013-2015.

Christchurch and Tohoku transformed our understanding of liquefaction

In the 2011 earthquakes in Christchurch, New Zealand, and Tohoku, Japan, liquefaction was extremely widespread, in some cases, devastating entire neighborhoods. However, an outgrowth of these events is a much better understanding about where liquefaction can happen, when it can happen, and what it can do. In fact, Christchurch residents became some of the most quake-savvy people in the world.

Forget about setting the parking brake; these cars are going nowhere. Christchurch neighborhoods like this one became unrecognizable due to massive liquefaction in 2011. As the water drains after the quake, the slurry congeals into cement, making its removal difficult and expensive. Photo courtesy of Stuff.co.nz

At Temblor, we want to people to better prepare themselves for what could come to pass. Perhaps one reason why earthquakes are not often discussed is fear. The idea of a magnitude-9 Cascadia earthquake off the coast of Oregon and Washington, or a magnitude-8 along the San Andreas Fault scares us all. So, in several blog posts, I will attempt to shed light on what liquefaction is, using my former home of Christchurch, New Zealand as a case study of what it is like to live with and be surrounded by liquefaction.

At 4:35 a.m. on September 4, 2010, Christchurch was rattled by a magnitude-7.1 earthquake. Over the next 4 years, over 10,000 aftershocks of varying size shook the city, most notably a magnitude-6.3 quake on February 22, 2011 which left 185 people dead, and large sections of the city in ruin. The February earthquake was so devastating in part because the epicenter was shallow (5 km or 3 mi) and located a few miles outside the city, meaning urban levels of shaking were some of the highest ever recorded, and the liquefaction was among the most profound and widespread ever experienced. However, Christchurch did not liquefy in every earthquake, and large sections of the city were left nearly unharmed by the earthquakes, making driving through the city an eerie experience. Nevertheless, liquefaction did occur on at least 8 occasions throughout the city. Additionally, in the magnitude-5.7 February 14, 2016 Valentine’s Day Earthquake, Christchurch one again experienced liquefaction. This not only highlighted how long earthquake sequences can last, but brought more damage to the still recovering city.

When liquefaction occurs, structures such as this Christchurch garage can sink and tilt. Photo courtesy of Becker Fraser Photos.

Temblor seeks to make it easy to learn where the liquefaction potential is high

In this Temblor map, areas deemed by the USGS to have very high liquefaction susceptibility in northern San Francisco are sites of fill (artificial land); the South of Market (SoMa) area was formerly a marsh and Mission Bay. All suffered liquefaction in the 1906 and 1989 earthquakes.Temblor recently added liquefaction susceptibility maps for all of Oregon and Washington. These maps illustrate how large portions of Seattle and Portland are susceptible to liquefaction. More Temblor maps of additional areas will be going up shortly.Houses that tilted and sunk in the 1906 earthquake due to liquefaction (Courtesy of the Bancroft Library, U.C. Berkeley)

In the next blog post, I will delve into some of the science behind liquefaction, including the research I was involved in during my 3 years in Christchurch. If anyone has questions or comments regarding these blogs or would like to be directed towards liquefaction data and/or research, post in the comments below and I will get back to you as soon as possible.

I would like you to cover, how deep can liquefaction go into the ground. Is it only the surface or maybe both 10 or 20 meter deep? And how do you prevent buildings from damage? Do you use Piling? Thanks for an interesting blog.

David Jacobson

That’s a great question. In most cases, the depth of liquefaction reaches down 2-3 meters. So, it is largely a near-surface event. However, this is dependent on how thick liquefiable layers are. In some cases in Christchurch, liquefaction occurred down to 8 meters. These were extreme cases, but they can nonetheless happen.

As for preventing buildings from damage, it is tricky. Ideally, you would avoid building on liquefiable soil. However, because many major cities have already been built on liquefiable soil, other methods can be attempted. Two of these are to make the soil denser by compacting it or to improve drainage qualities. Additionally, it is possible to build a structure that could possibly be liquefaction resistant. This would involve either shallow or deep foundation elements with ductile features making them able to withstand significant motion. While these methods can be attempted, it is possible they will not work, as liquefaction is extremely site-specific and can effect the surface in a variety of ways.

If you would like any more information about this, feel free to ask and I can direct you to additional data.

Vance Adams

I live in Oregon. I have an Oregon earthquake policy and it does not specifically state that it covers liquefaction. Infact, most everyone I’ve been able to speak with for several different insurance companies say the policy appears to exclude soil liquefaction. I looked at a policy template from CA and it specifically includes soil liquefaction as a covered risk. I’ve spent hours and hours on the phone and no one can give me a definative answer as to whether or not Oregon earthquake policies cover soil liquefaction. Is there anyone that could track this down for an answer from an authoritative source? I’m planning to move just because I can’t find out if I have coverage or not. – Vance

David Jacobson

Unfortunately I do not know of anyone that you would be able to contact outside of the insurance companies. It is possible that because liquefaction hasn’t occurred much in the Northwest that insurance companies haven’t included it in their policies because they are unaware of the risks. Also, I want to stress that even if you live in a liquefaction-prone area, it does not necessarily mean that liquefaction will occur in an earthquake, as liquefaction is extremely site-specific.

Gary

What are your comments for liquifaction in the coastal Newport Beach, Huntington Beach, California areas. Any accurate maps of the affected areas available?

David Jacobson

There are some very generalized maps on the Temblor website of the Newport and Huntington beach areas. However, these don’t have much coverage. Additionally, the maps available in Southern California are designed to show if soils are susceptible rather than illustrating how susceptible. I will take a look at some of the geologic maps of the area since that can help give a rough estimation.

David Jacobson

After looking at some geologic maps and going back through additional county data, it seems likely these area would be relatively susceptible to liquefaction. In order for liquefaction to occur, sediments must be relatively young, water-saturated and experience a certain level of ground shaking (I will go into more detail in my next blog post). Coastal areas tend to fit these criteria. I am including a link to an Orange County liquefaction map. While rather basic, it should give you a good idea of what the relative susceptibilities are in the area. If you want any more information, feel free to ask.http://www.mwdoc.com/cms2/ckfinder/files/files/Fig4_3-3_Liquefaction%20Map.pdf